Question

a. Explain the primary purpose of regeneration in a Rankine power plant. Sketch a T-s diagram using either open or closed feedwater heater to explain your response. 

b. Explain the advantages and disadvantages of open and closed feedwater heaters. 

c. Assume steady-flow, no heat transfer, no work, one entrance, and one exit and neglect elevation changes. Show that the stagnation enthalpy is constant throughout a nozzle. 

d. Recall that the pressure ratio that maximizes the net work for the simple Brayton cycle and makes T4 = T2. What happens if the regenerative Brayton cycle operates at a pressure ratio larger than this ratio? The pressure ratio that maximizes the net work is given by: 

e. Explain why the assumption that the water vapor is an ideal gas is valid when the mixture temperature is below 50°C. 

Answer 1

1. Regeneration improves the cycle efficiency by increasing the initial feed water temperature before the water enters the boiler and also helps in controlling the large flow rate of steam at the turbine exit.The energy required in the boiler would then decrease leading to increase in efficiency.

5 ) 2' 13 < 2 7 S 1-2-3-4-5-6-7'- Regenerative Rankine 1 - 2 -2-4-5-6-7 - Simple Rankine We Can See 4-5 and 2²-5 is the process in boiler. Q required in Regenerative is less compared to that in "Simple Rankine Hence n increases

2 Open FWH closed FWA 1. Extracted steam is Mixed with feed Water in heater. 1 Extracted steam is not mixed with feed Waten. & Design is simple 3. Less Costly 2. Design is complex 3. More Cos Costly 4. Need larger Area 4. requires less Area 5. Open feed Water heaters requires a pump for each 5. closed EWH require only a Single pump for the main feed water How regardless of number heaters heater for handling the feed water flow of 921 + g = ha + (3) hi + viz 2000 1900 h + f +972 + th 2 pood igoo NO Work, NO heat > 8 and W=0 Elevation changes NO VNV2

h he Enthalpy Hed Remains Constant